Mode select circuit

ABSTRACT

In accordance with the present invention a mode select circuit includes a bias circuit and a voltage level encoder. The mode select circuit further includes a mode select terminal capable of being selectively coupled to one or more of a plurality of configuration elements to bias the mode select terminal to one of a plurality of predesignated voltages. The bias circuit is coupled to the mode select terminal for biasing the terminal to one of the predesignated voltages when the mode select terminal is not coupled to any of the configuration elements. The voltage level encoder is coupled to the mode select terminal for providing one of a plurality of voltage level codes on a plurality of voltage level encoder output terminals in response to the mode select terminal being biased to a corresponding one of the plurality of predesignated voltages.

BACKGROUND OF THE INVENTION

This invention relates to integrated circuits and more particularly to a circuit for configuring an integrated circuit to operate in one or more of a number of different modes of operation.

Integrated circuits (ICs) are typically required to operate in a variety of different modes. For example, microprocessors may be configured to run in either high speed or low speed mode, or alternatively, high power or low power mode. Other ICs, such as analog ICs, may be configured to one of a number of modes to change the characteristics of the device. For example, mode selection in an analog system may change the type of signal processing carried out by the analog system or it may change the internal electrical characteristics of the individual analog functional blocks.

Conventional mode selection circuits are typically implemented using alterable memory structures such as DRAM, SRAM, EEPROM, or Flash, which are integrated on a single semiconductor chip with other portions of an IC. One problem with such structures is that they require special semiconductor processing technologies that may not be readily available or otherwise may increase manufacturing cost. Moreover, the required circuitry for programming such memory structures consumes silicon area, resulting in further increase in cost.

Another conventional method for implementing mode selection consists of hardwiring a device pin to either supply or ground. However, this technique has the disadvantage of only allowing for two modes per pin. Therefore, unless multiple pins are used, only two modes will be available. The use of multiple pins to implement additional modes increases packaging costs as well as the size of the IC.

In some classes of electronic systems, the size and cost of the ICs can be critical and must be minimized. Accordingly, the manufacturing process must be simple and the method of mode selection must be direct. Thus, a simple and cost effective mode selection circuit technique capable of selecting one of at least three operating modes, and which can be implemented in simple or complex process technologies is desired.

SUMMARY OF THE INVENTION

In accordance with the present invention a mode select circuit includes a voltage level encoder and a mode select terminal capable of being selectively coupled to one or more of a plurality of configuration elements to bias the mode select terminal to one of a plurality of predesignated voltages. The voltage level encoder is coupled to the mode select terminal for providing one of a plurality of voltage level codes on a plurality of voltage level encoder output terminals in response to the mode select terminal being biased to a corresponding one of the plurality of predesignated voltages.

In one embodiment, the mode select circuit further includes a bias circuit coupled to the mode select terminal for biasing the terminal to one of the predesignated voltages when the mode select terminal is not coupled to any of the configuration elements.

In another embodiment, each of the plurality of voltage level codes configures a first integrated circuit to operate in a respective one of a plurality of operating modes.

In another embodiment, the mode select circuit is in the first integrated circuit.

In another embodiment, the mode select circuit is housed in a second integrated circuit coupled to the first integrated circuit.

In another embodiment, the configuration elements are external to an integrated circuit in which the mode select circuit is housed.

In another embodiment, the voltage level encoder comprises a plurality of voltage detectors each having a trip point, the trip points of the voltage detectors being offset one from another to define a plurality of voltage ranges. Each of the plurality of predesignated voltages is within a respective one of the plurality of voltage ranges.

In another embodiment, the mode select circuit further includes a first power supply terminal for receiving a power supply voltage source and a second power supply terminal for receiving a reference potential, the trip point of each of the voltage detectors being maintained across a range of voltages closer to the ground potential than the power supply voltage.

In yet another embodiment, a mode select circuit includes a bias circuit and a voltage level encoder. The mode select circuit further includes a mode select terminal capable of being selectively coupled to one or more of a plurality of configuration elements to bias the mode select terminal to one of a plurality of predesignated voltages. The bias circuit is coupled to the mode select terminal for biasing the terminal to one of the predesignated voltages when the terminal is not coupled to any of the configuration elements. The voltage level encoder includes a plurality of comparators each having a first input terminal coupled to the mode select terminal and a second terminal coupled to one of a plurality of reference voltages. The voltage level encoder provides one of a plurality of voltage level codes on a plurality of voltage level encoder output terminals in response to the mode select terminal being biased to a corresponding one of the plurality of predesignated voltages.

In another embodiment, he mode select circuit further includes a first power supply terminal for receiving a power supply voltage source, a second power supply terminal for receiving a reference potential, and a plurality of resistors serially connected between the first and second power supply terminals. An intermediate node between every two adjacent resistors provides the plurality of reference voltages.

In another embodiment, each of the serially connected resistors defines a voltage range, each of the plurality of predesignated voltages being within a respective one of the plurality of voltage ranges.

The following detailed description and the accompanying drawings provide a better understanding of the nature and advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a mode select circuit in accordance with an embodiment of the present invention.

FIG. 2 is a first example of a circuit implementation of the FIG. 1 block diagram, illustrating a mode select circuit capable of configuring an integrated circuit in one of three different modes.

FIG. 3 is a second example of a circuit implementation of the FIG. 1 block diagram, illustrating a mode select circuit capable of configuring an integrated circuit in one of N+1 different modes.

FIG. 4 illustrates an example of a more detailed circuit implementation of the mode select circuit of FIG. 2.

FIG. 5 illustrates a switch controller application of the mode selection technique of the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

FIG. 1 shows a block diagram of a mode select circuit 100 in accordance with one embodiment of the present invention. Mode select circuit 100 includes a voltage level encoder 110 coupled to mode select logic 120 via terminals 115. Voltage encoder 110 is also coupled to a mode select input node 101 which can selectively be coupled to a plurality of configuration elements 150. Configuration elements 150 includes power supply voltage Vdd 151, ground potential GND 152, and resistor R 153 coupled to ground. Each of the configuration elements bias node 101 to a different voltage when coupled to node 101. Node 101 is biased to a fourth voltage by a bias circuit (not shown) internal to voltage level encoder 110 (i.e., none of the configuration elements are coupled to node 101).

Mode select circuit 100 is capable of programming the functional blocks of an integrated circuit (“IC”) into one of four different modes by biasing node 101 to a corresponding one of the four different voltages. The IC enters a first mode when node 101 is biased substantially close to Vdd by coupling Vdd element 151 to mode select input node 101. The IC enters a second mode when node 101 is biased substantially close to GND by coupling GND element 152 to node 101. The IC enters a third mode when node 101 is biased to a first voltage between GND and Vdd by coupling element R 153 to node 101. The IC enters a fourth mode when node 101 is biased to a second voltage between GND and Vdd by bias circuitry internal to voltage level decoder 110 (i.e., node 101 is not coupled to configuration elements 150).

Mode select circuit 100 operates as follows. After biasing node 101 to one of the four voltages, voltage level encoder 110 determines which of a plurality of voltage ranges the voltage on node 101 is within. Voltage level encoder 110 then generates a voltage level code on terminals 115 corresponding to the voltage range within which the node 101 voltage is. Mode select logic 120 receives the voltage level code via terminals 115, and processes the voltage level code to produce a mode select code on output terminals 125. The mode select code is transmitted to the functional blocks of the IC configured the IC into one of a plurality of modes corresponding to the particular mode select code.

Although FIG. 1 shows selection of one of four operating modes, the present invention is not limited as such. FIGS. 2 and 3 show exemplary embodiments (i.e., variations of mode select circuit 100 of FIG. 1) wherein fewer than four modes (FIG. 2) and greater than four modes (FIG. 3) are made available for selection.

FIG. 2 is a first example of a circuit implementation of the FIG. 1 block diagram, illustrating a mode select circuit 200 capable of configuring an IC into one of three different modes. Mode select circuit 200 includes a voltage bias circuit 202 coupled to a mode select input node 201 and to an input of a voltage level detection circuit 204. Mode select circuit 200 further includes a mode select logic 220 having inputs coupled to the outputs of detection circuit 204. Also shown in FIG. 2 are configuration elements 250 capable of being selectively coupled to node 201.

In the first mode, voltage source Vdd element 251 is coupled to mode select input node 201 to bias node 201 to a voltage substantially close to Vdd. In the second mode, a ground potential GND element 252 is coupled to mode select input node 201 to bias node 201 to a voltage substantially close to GND. Finally, in the third mode, mode select input node 201 is not coupled to configuration elements 250, and is instead biased by internal bias circuit 202 to a voltage Vbias between Vdd and GND. Accordingly, mode select input node 201 can be biased to one of three voltages Vdd, GND, or Vbias corresponding to three different configuration modes.

In the FIG. 2 embodiment, the internal bias circuit 202 is a resistor divider which includes a first resistor 210 and a second resistor 212 coupled in series between Vdd and ground. The resistor divider sets up a DC bias voltage Vbias on bias node 211. Other circuit elements, such as transistors, can be used instead of or in combination with resistors 210, 212 to generate the desired bias voltage on bias node 211, as is well-known by those skilled in the art.

Voltage level detection circuit 204 senses the voltage on input node 201 and generates a corresponding voltage level code on its output terminals 216 and 217. Voltage level detection circuit 204 includes two voltage detectors 214 and 215, each having a unique trip point. When a voltage on the input of a voltage detector is below its trip point the detector generates a first signal, and when the voltage is above the detector's trip point the detector generates a second (e.g., opposite) signal. In the FIG. 2 embodiment, the voltage detectors are inverters INV1 214 and INV2 215 having their input terminals connected to node 211, and their respective output terminals 216 and 217 providing signals A and B.

The trip points of the two inverters are offset one from the other such that their output signals switch at different input voltages. In the FIG. 2 embodiment, INV1 has a lower trip point than INV2, although the reverse may also be implemented. The two inverters with trip points offset one from the other establish three voltage ranges. The first range is from ground potential to the first trip point within which both CMOS inverters will output a high (e.g. logic ‘1’). The second range is from the first trip point to the second trip point within which INV1 outputs a low (e.g. logic ‘0’), and INV2 outputs a high. The third and final range is above the second trip point, within which both INV1 and INV2 output a low. Thus, output signals A and B of INV1 and INV2 together form a voltage level code that indicates which of the voltage ranges defined by the offset trip points the mode select input node is in.

The voltage level code A, B is transmitted to mode select logic 220 via terminals 216 and 217. Mode select logic 220 processes voltage level code A, B to produce a mode select code M1, M2, M3 that is used to control the functional blocks of the IC. Mode select logic 220 includes 2-input NAND gates 221 and 222, inverters 223 and 225, and 2-input NOR gate 224. These gates are connected to form a logic block which provides a unique mode select code M1, M2, M3 for each voltage level code A, B.

Table 1 below illustrates the relationship between the configuration elements 250, the voltage level code A, B, and the mode select code M1, M2, M3. For each configuration element connected to input node 201 and the case where node 201 is biased solely by bias circuit 202, there is a corresponding unique mode select code M1, M2, M3. Each mode select code has a logic ‘0’ only in the bit location corresponding to the particular mode (e.g., for mode 3, only M3 is logic ‘0’).

TABLE 1 Voltage Level Code Mode Select Code Node 201 A B M1 M2 M3 Vdd 0 0 1 1 0 element Bias Circuit 0 1 1 0 1 202 only GND 1 1 0 1 1 element

The FIG. 2 embodiment is not limited to such mode select code designation, and other mode select code designations can also be used. Additionally, the FIG. 2 embodiment illustrates a mode select code that operates on parallel lines 226, 227, and 228. However, other methods of transmitting the mode select code, for example serial mode control signals, could also be used. In one embodiment, the functional blocks of the IC are controlled directly by the voltage level code A, B.

FIG. 3 is a second example of a circuit implementation of the FIG. 1 block diagram, illustrating a mode select circuit 300 capable of configuring an IC into one of N+1 modes, N being a positive integer. Mode select circuit 300 includes a voltage bias circuit 302 coupled to a mode select input node 301 and to an input of a voltage level detection circuit 304. Mode select circuit further includes a mode select logic 320 having inputs coupled to the outputs of detection circuit 304. Voltage bias circuit 302 is a voltage divider including resistors 310 and 312 serially connected between Vdd and ground.

Voltage level detection circuit 304 includes N two-input comparators 331-(1) to 331-(N), and a resistor network 370 that includes N+1 tap resistors 332-(1) to 332-(N+1) serially-connected between Vdd and ground for setting a unique offset trip point for each comparator. In one embodiment, the two-input comparator circuits are analog amplifiers or analog comparators.

Resistor network 370 defines N+1 mutually exclusive voltage ranges. Each voltage range is defined by the voltage drop across each resistor in resistor network 370 (e.g., a first voltage range defined by resistor 332-(1) equals the range between Vdd and V1 at node 334-(1), and a (N+1)th voltage range defined by resistor 332-(N+1) equals the range between Vn at node 334-(N) and V_(GND). Thus, the N+1 voltage ranges are set by selecting the desired values of the resistors in resistor network 370 and tapping off a voltage from nodes between every two adjacent resistors. The voltage taps between each two adjacent resistors are coupled to the negative input terminals of the comparators. Other bias circuits capable of generating a plurality of voltage ranges could also be used instead of resistor network 370 without departing from the spirit and scope of the present invention.

Mode select circuit 300 is capable of configuring an IC into one of N+1 different operating modes by selectively coupling input node 301 to one of N configuration elements (not shown) and including allowing biasing node 301 to be biased solely by bias circuit 302. Similar to the configuration elements 150 and 250 in FIGS. 1 and 2, each of the N configuration elements used with mode select circuit 300 biases node 301 to N distinct voltages. Resistors of different values coupled to Vdd and ground may be used to implement some or all of the N configuration elements.

As an example, coupling node 301 to a configuration element Vdd biases node 311 substantially close to Vdd. This sets the positive input terminal of all the comparators to substantially close to Vdd. Resistor network 370 is designed such that the voltage at node 301 falls within a first voltage range defined by the voltage drop across resistor 331-(1) (e.g., the voltage range between Vdd and V1 at node 334-(1)). Thus, the positive input terminal of each of the comparators is at a greater voltage than all the negative input terminals. All the comparators thus output a high (logic ‘1’). In this manner, the voltage level code on output nodes A, B, C, . . . , N of the comparators is {1, 1, 1, . . . , 1} corresponding to a first mode.

As another example, another configuration element (not shown) coupled to node 301 biases node 301 to a first bias voltage which falls within a second voltage range defined by the voltage drop across resistor 331-(2) (e.g., the voltage range between V1 at node 334-(1) and V2 at node 334-(2)). Voltage V1 at the negative input terminal of comparator 331-(1) is thus higher than the first bias voltage, while the voltages at the negative input terminals of all the other comparators is lower than the first bias voltage. Therefore, output A of comparator 331-(1) is low, and outputs B, C, . . . , N of the other comparators are high. Accordingly, the voltage level code on output nodes A, B, C, . . . , N is {0, 1, 1, . . . , 1} corresponding to a second mode.

As can be seen from the above examples, by selecting proper values for the resistors in resistor network 370 and using properly designed configuration elements, detection circuit 304 provides a unique binary combination at its output terminals for each configuration element. Detection circuit 304 also provides a unique binary combination when no configuration element is coupled to node 301, and node 301 is biased solely by bias circuit 302. This corresponds to one of the N+1 modes.

The output terminals of comparators 331-(1) to 331-(N) are coupled to the input terminals of mode select logic 320 to produce a mode select code {M1, M2, M3, M4, . . . , M(N+1)} at the output terminals of mode select logic 320. Mode select logic 320 includes N two-input NOR gates and N-1 inverters. Each NOR gate provides one of the M signals at its output terminal. The upper input terminal of NOR gate 335-(2) is coupled to output terminal B of comparator 331-(2) and to the input of inverter 333-(1), and the lower input terminal of NOR gate 335-(2) is coupled to an output of inverter 333-(2). All other NOR gates between NOR gate 335-(1) and NOR gate 335-(N) are similarly connected. The upper input terminal of NOR gate 335-(1) is coupled to output terminal A of comparator 331-(1) which also provides signal M1. The lower in put terminal of NOR gate 335-(1) is coupled to an output terminal of inverter 333-(1). The upper input terminal of NOR gate 335-(N) is coupled to output terminal N of comparator 331-(N) and to an input of inverter 333-(N-1) not shown. The lower input terminal of NOR gate 335-(N) is grounded.

For each binary combination produced at the output terminals of detection circuit 304, mode select logic 320 provides a high at only one of its output terminals. This is shown more clearly in Table 2. Table 2 illustrates the voltage level code {A, B, . . . , N} and mode select code {M1, M2, . . . , M(N+1)} corresponding to each of the configuration elements and the case of node 301 being biased solely by bias circuit 302. Letter “S” in each row represents a distinct biasing voltage corresponding to one of the N configuration elements and the case of node 301 being biased by bias circuit 302 alone.

TABLE 2 Voltage Level Code Mode Select Code A B . . . N M1 M2 M3 . . . M(N + 1) S > V1 (S = Vdd) 1 1 . . . 1 1 0 0 . . . 0 V2 < S < V1 0 1 . . . 1 0 1 0 . . . 0 V3 < S < V2 0 0 . . . 1 0 0 1 . . . 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S < Vn (S = Gnd) 0 0 . . . 0 0 0 0 . . . 1

In Table 2, opposite polarity logic for the voltage level code could be achieved by simply switching the inputs of the comparators.

FIG. 4 illustrates an example of a more detailed circuit implementation of the mode select circuit of FIG. 2. Mode select circuit 400 of FIG. 4 includes a voltage level encoder 450 coupled to a mode select logic 420 and a mode select input node 401. Voltage level encoder 450 includes a voltage bias circuit 402 and a voltage level detection circuit 404.

Mode select circuit 400 selects of one of three modes M1, M2, M3 by either coupling mode select input node 401 to one of two configuration elements (not shown) or biasing node 401 solely by circuitry internal to circuit 400. In one embodiment, the configuration elements include a voltage source Vdd for biasing node 401 to a first voltage, and a ground potential GND for biasing node 401 to a second voltage. Mode select input node 401 can be biased to a third voltage by not coupling the configuration elements to node 401, thus allowing node 401 to be biased by bias circuit 402 only. Bias circuit 402 is a resistor divider comprised of serially-connected resistors 410 and 412.

The three different biasing voltages on node 401 are encoded by voltage level encoder 450 according to three voltage ranges defined by the trip points of two voltage detectors 414 and 415. Voltage detector 414 is a depletion load inverter which includes a native NMOS transistor 431 (e.g., an NMOS transistor that receives no threshold voltage adjustment during processing, thus having a threshold voltage of for example 0.2V) and a depletion NMOS transistor 430 connected as a load device. Voltage detector 415 is a source degenerated CMOS inverter which includes NMOS transistor 433 and PMOS transistor 432 with their gates coupled to node 401. Inverter 415 further includes a biasing device connected to the source of NMOS transistor 433, which in this embodiment is a depletion NMOS transistor 434 with its gate and source connected together. Other biasing devices such as current mirrors or resistors could also be used.

Depletion load inverter 414 and source degenerated inverter 415 together form the voltage level detection circuit 404 for producing a voltage level code A, B corresponding to one of the three biasing voltages on node 401. Inverter 414 has a relatively low trip point (e.g., in the range of 0.2V to 0.5V) achieved primarily by using the native NMOS transistor 431. Inverter 415 has a higher trip point (e.g., in the range of 1V to 2V) achieved primarily by ratioing of transistor device sizes and the use of biasing transistor 434. The trip points of inverters 414 and 415 define three voltage ranges: The first voltage range equals the range between Vdd and inverter 415 trip point; the second voltage range equals the range between the two inverter trip points; and the third voltage range equals the range between the voltage corresponding to inverter 414 trip point and GND.

When node 401 is coupled to configuration element Vdd (not shown), node 401 gets biased to a voltage substantially close to Vdd. This voltage falls within the first voltage range and is thus greater than the trip points of inverters 414 and 415. Accordingly, inverters 414 and 415 provide a low at their respective outputs. The voltage level code A, B is thus {0, 0} which corresponds to a first mode.

When node 401 is biased solely by bias circuit 402 (i.e., node 401 is not coupled to any configuration elements), the resistor divider in bias circuit 402 biases node 401 to a voltage within the second voltage range. Thus, the node 401 voltage is less than the inverter 415 trip point but greater than the inverter 414 trip point. Accordingly, inverter 415 provides a high at its output and inverter 414 provides a low at its output. The voltage level code A, B is thus {0, 1} which corresponds to a second mode.

When node 401 is coupled to configuration element GND (not shown), node 401 gets biased to a voltage substantially close to ground potential. This voltage falls within the third voltage range and is thus lower than the trip points of inverters 414 and 415. Accordingly, inverters 414 and 415 provide a low at their respective outputs. The voltage level code {A, B } is thus {1, 1} which corresponds to a third mode.

The outputs of inverters 414 and 415 are coupled to mode select logic 420 via transmission gate 490 and buffer inverters 480, 481, respectively. Mode select logic 420 processes the voltage level code {A, B} to produce a mode select code {M1, M2, M3}. Mode select logic 420 includes two-input NAND gates 421 and 422, inverters 423 and 425, and NOR gate 424. These interconnected gates form a logic block which provides a unique mode select code {M1, M2, M3} for each voltage level code {A, B}. Table 3 illustrates the voltage level code and mode select code corresponding to each configuration element and the case of node 401 being biased solely by bias circuit 402.

TABLE 3 Voltage Level Code Mode Select Code Node 401 A B M1 M2 M3 Vdd 0 0 1 1 0 element Bias Circuit 0 1 1 0 1 402 only GND 1 1 0 1 1 element

Mode select circuit 400 also includes circuitry for powering down mode select circuit 400. This circuitry includes a CMOS inverter comprised of transistors 471 and 472, a supply decoupling transistor 473, a transmission gate comprised of transistors 474 and 475, bias decoupling transistor 476, discharge transistor 477, and mode select logic shutdown transistor 478. These transistors serve to cut off DC current paths in mode select circuit 400 when circuit 400 is powered down (i.e., signals EN and {overscore (EN)} are set to low and high respectively). Transistors 477 and 478 also serve to set mode select code {M1, M2, M3} to a different code than the three codes used during normal operation.

In FIG. 4, the output of source degenerated inverter 415 is coupled to two serially connected buffer inverters 480 and 481. The buffer inverters provide voltage gain to ensure that node B signals swing rail to rail (i.e., Vdd to ground). It should be noted that for the embodiment shown in FIG. 4, the circuit configuration of inverters 414, 415 and the choice of resistor ratio in bias circuit 402 provides proper operation over a wide range of supply voltage Vdd.

FIG. 5 illustrates a switch controller application for the mode selection technique of the present invention. FIG. 5 shows a MOSFET control circuit 500 for controlling a P-type MOSFET 503 which drives a load 505. MOSFET 503 is designed to deliver a large current. A switch controller 501 coupled to MOSFET 503 controls voltage slew rate at the gate of MOSFET 503 to limit inrush current to a large capacitance component of load 505. MOSFET control circuit 500 is particularly advantageous in applications where the size of load 505 is not known in advance. Depending on the size of load 505, the desired speed for charging load 505, and the tolerated Vdd power surges, MOSFET control circuit 500 can be configured to operate in one of a number of modes to provide a turn-on signal with an optimum slew rate at the gate of MOSFET 503.

Switch controller 501 generates a gate control signal for turning on and off MOSFET 503 in response to a switch control input signal 502. MOSFET 503 has a source connected to a supply voltage 504, a drain connected to a load 505, and a gate coupled to receive the gate control signal from switch controller 501. Additionally, control circuit 500 includes a NMOS transistor 506 for quickly turning off MOSFET 503 in response to switch control signal 502 providing a signal for turning off MOSFET 503.

Switch controller 501 further includes a mode select pin 507 which can be left floating (i.e., be biased solely by bias circuitry internal to switch controller 501) or be coupled to one of two configuration elements Vdd 504 and ground potential GND 512. As in previous embodiments, each of the three pin 507 biasing conditions places control circuit 500 in one of three modes. Thus, switch controller 500 can be configured into one of three different modes corresponding to different slew rates for turning on MOSFET 503.

In one embodiment, in the first mode wherein node 507 is coupled to Vdd 504 via jumper J1, switch controller 501 provides the fastest slew rate; in the second mode wherein node 507 is biased solely by circuitry internal to switch controller 501, switch controller 501 provides a slower slew rate than the first mode; and in the third mode wherein node 507 is coupled to ground 512 via jumper J2, switch controller 501 provides the slowest slew rate. Note that fewer or more than three modes can be provided for generating fewer or more slew rates as described previously.

While various preferred and alternate embodiments of the present invention have been disclosed and described in detail, it should be evident that the present invention is equally applicable by making appropriate modifications to the embodiments described above. In other instances, well-known electrical structures and circuits are shown in block diagram form in order not to unnecessarily obscure the present invention. For example, in one embodiment, each of the circuits shown in FIGS. 1-4 may form part of the IC that it reconfigure, and in an alternate embodiment, each of the circuits of FIGS. 1-4 are in one IC which is used to reconfigure another IC.

Therefore, the above description should not be taken as limiting the scope of the invention which is defined by the metes and bounds of the appended claims. 

What is claimed is:
 1. A mode select circuit comprising: a mode select terminal capable of being selectively coupled to one or more of a plurality of configuration elements to bias the mode select terminal to one of a plurality of predesignated voltages; a bias circuit coupled to the mode select terminal for biasing the terminal to one of the predesignated voltages when the terminal is not coupled to any of the configuration elements; and a voltage level encoder coupled to the mode select terminal for providing one of a plurality of voltage level codes on a plurality of voltage level encoder output terminals in response to the mode select terminal being biased to a corresponding one of the plurality of predesignated voltages.
 2. The mode select circuit of claim 1 wherein each of the plurality of voltage level codes configures a first integrated circuit to operate in a respective one of a plurality of operating modes.
 3. The mode select circuit of claim 2 wherein the mode select circuit is in the first integrated circuit.
 4. The mode select circuit of claim 2 wherein the mode select circuit is housed in a second integrated circuit coupled to the first integrated circuit.
 5. The mode select circuit of claim 1 wherein the configuration elements include a DC voltage source and a resistor.
 6. The mode select circuit of claim 1 wherein the configuration elements are external to an integrated circuit in which the mode select circuit is housed.
 7. The mode select circuit of claim 1 further comprising a mode select logic block coupled to the plurality of encoder output terminals for providing a unique mode select code in response to each of the plurality of voltage level codes.
 8. The mode select circuit of claim 2 wherein the first integrated circuit includes a MOSFET switch controller coupled to receive the mode select codes for controlling the rate at which a MOSFET is turned on or off.
 9. The mode select circuit of claim 8 wherein in response to each mode select code the MOSFET switch controller provides a signal having a corresponding ramp rate at a gate of the MOSFET for turning on the MOSFET.
 10. The mode select circuit of claim 1 wherein the voltage level encoder comprises a plurality of voltage detectors each having a trip point, the trip points of the voltage detectors being offset one from another to define a plurality of voltage ranges.
 11. The mode select circuit of claim 10 wherein each of the plurality of predesignated voltages is within a respective one of the plurality of voltage ranges.
 12. The mode select circuit of claim 10 further comprising a first power supply terminal for receiving a power supply voltage source and a second power supply terminal for receiving a reference potential, the trip point of each of the voltage detectors being maintained across a range of voltages closer to the reference potential than the power supply voltage.
 13. The mode select circuit of claim 10 wherein the voltage detectors include a depletion load NMOS inverter.
 14. The mode select circuit of claim 10 wherein the voltage detectors include a CMOS inverter.
 15. The mode select circuit of claim 10 wherein the voltage level detectors comprise: a depletion load NMOS inverter coupled to the mode select terminal; and a source degenerated CMOS inverter coupled to the mode select terminal, wherein the source degenerated CMOS inverter has a trip point that is higher than the trip point of the depletion load NMOS inverter.
 16. The mode select circuit of claim 2 wherein the first integrated circuit is configurable to operate in one of at least three modes, the first integrated circuit being configured to operate in a first mode if the mode select terminal is coupled to a DC voltage source, the first integrated circuit being configured to operate in a second mode if the mode select terminal is coupled to a ground potential, and the first integrated circuit being configured to operate in a third mode if the mode select terminal is biased to said one of the plurality of predesignated voltages.
 17. The mode select circuit of claim 16 wherein the first integrated circuit is configured to operate in a fourth mode if the input terminal is coupled to a reference voltage through an external resistor.
 18. A mode select circuit comprising: a mode select terminal capable of being selectively coupled to one or more of a plurality of configuration elements to bias the mode select terminal to one of a plurality of predesignated voltages; a bias circuit coupled to the mode select terminal for biasing the terminal to one of the predesignated voltages when the terminal is not coupled to any of the configuration elements; and a voltage level encoder comprising a plurality of comparators each having a first input terminal coupled to the mode select terminal and a second terminal coupled to one of a plurality of reference voltages, the voltage level encoder providing one of a plurality of voltage level codes on a plurality of voltage level encoder output terminals in response to the mode select terminal being biased to a corresponding one of the plurality of predesignated voltages.
 19. The mode select circuit of claim 18 further comprising: a first power supply terminal for receiving a power supply voltage source; a second power supply terminal for receiving a reference potential; and a plurality of resistors serially connected between the first and second power supply terminals, an intermediate node between every two adjacent resistors providing the plurality of reference voltages.
 20. The mode select circuit of claim 19 wherein each of the serially connected resistors defines a voltage range, each of the plurality of predesignated voltages being within a respective one of the plurality of voltage ranges.
 21. The mode select circuit of claim 18 wherein each of the plurality of voltage level codes configures a first integrated circuit to operate in a respective one of a plurality of operating modes.
 22. The mode select circuit of claim 21 wherein the mode select circuit is in the first integrated circuit.
 23. The mode select circuit of claim 21 wherein the mode select circuit is housed in a second integrated circuit coupled to the first integrated circuit.
 24. The mode select circuit of claim 18 wherein the configuration elements are external to an integrated circuit in which the mode select circuit is housed.
 25. The mode select circuit of claim 21 further including a MOSFET switch controller coupled to receive the voltage level codes for controlling the rate at which a MOSFET is turned on or off.
 26. The mode select circuit of claim 25 wherein in response to each voltage level code the MOSFET switch controller provides a signal having a corresponding ramp rate at a gate of the MOSFET for turning on the MOSFET.
 27. The mode select circuit of claim 21 wherein the first integrated circuit is configurable to operate in one of at least three modes, the first integrated circuit being configured to operate in a first mode if the mode select terminal is coupled to a DC voltage source, the first integrated circuit being configured to operate in a second mode if the input terminal is coupled to a ground potential, and the first integrated circuit being configured to operate in a third mode if the input terminal is biased to said one of the plurality of predesignated voltages.
 28. The mode select circuit of claim 27 wherein the first integrated circuit is configured to operate in a fourth mode if the input terminal is coupled to a reference voltage through an external resistor.
 29. A method of configuring an integrated circuit into one of a plurality of modes, comprising: selectively coupling one of a plurality of configuration elements to a mode select terminal to bias the mode select terminal to a corresponding one of a plurality of voltages; biasing the mode select terminal to a voltage provided by a bias circuit when none of the plurality of configuration elements are coupled to the mode select terminal; detecting the voltage on the mode select terminal; providing a voltage level code in response to the detected voltage; and configuring the integrated circuit into one of a plurality of operating modes corresponding to the provided voltage level code.
 30. The method of claim 29 further comprising: in response to the provided voltage level code, providing a signal having a predesignated ramp rate at a gate of a MOSFET for turning on the MOSFET.
 31. The method of claim 29 wherein the detecting step is carried out by a plurality of voltage detectors each having a trip point, the method comprising: providing a plurality of voltage ranges by offsetting the trip points of the plurality of voltage detectors, each of the predesignated voltage levels being within a respective one of the plurality of voltage ranges.
 32. The method of claim 29 further comprising: coupling a voltage source to the mode select terminal to configure the integrated circuit to operate in a first mode, the voltage source being external to the integrated circuit; coupling a ground potential to the mode select terminal to configure the integrated circuit to operate in a second mode, the ground potential being external to the integrated circuit; and biasing the mode select terminal to the voltage provided by the bias circuit internal to the integrated circuit to configure the integrated circuit to operate in a third mode.
 33. The method of claim 32 further comprising: coupling the input terminal to a reference voltage through a resistor to configure the integrated circuit to operate in a fourth mode, the reference voltage and the resistor being external to the integrated circuit. 